In some applications it is required that a high voltage or primary circuit die support one or more low voltage secondary dies so that the low voltage circuit dies are buffered from high voltages and can be made much more inexpensively. In one automotive application the primary circuit die interfaces with the 12 volt vehicle power supply while the secondary circuit die(s) interfaces only with the low voltage side of the primary circuit die.
Typical high voltage sensor interfaces in automotive applications are LIN, SENT or PWM. Each of these interfaces is based on an open collector output and an external pullup resistor (in practice will have additional circuits for buffering and emc). In response to a low voltage control signal the open collector device is hard on and pulls the output low or is off and lets the output float high. If the pullup resistor is to 12V then the open collector device and pullup resistor functions as a level shifter from typical digital logic voltage levels of 0, 3.3V to typical automotive voltage levels of 0 to 12V.
PWM and SENT interfaces are unidirectional and only transmit data. For a PWM the data is transmitted based on the pulse high or low time. e.g. if only transmitting temperature then the output format could be a 50 mS high time followed by a low time which is 10 mS+1 mS for every degree the temperature exceeds −40′c. If a PWM must transmit multiple parameters then many elaborate schemes are available such as one where a 50 mS high time is followed by low time representing the temperature as before and then a 100 mS high time followed by the second parameter represented by the low time. The high time therefore indicates to a knowledgeable receiver which parameter is being transmitted next.
The SENT (single edge nibble transmission) interface is described in SAE J2716. In is considered a lower cost version of CAN or LIN but with advantages over a PWM. Similar to a PWM it is unidirectional and only transmits data. The sensor signal is transmitted as a series of pulses measured from falling edge to falling edge. These include a calibration/synchronization pulse, 4 bit data pulses followed by a CRC pulse.
LIN (Local interconnect network) is a 3rd common low cost automotive interface and is controlled and specified by the LIN consortium. It is bidirectional with a master/slave organization where the master initiates all transactions. All slaves have a unique ID and the master interrogates individual slaves for the data it requires. In LIN this unique ID is referred to as a PID. The addressed slave responds with packets of data. The LIN physical interface is similar to that for the PWM and SENT interfaces being basically an open collector output with additional constraints on slew rates, pulse widths, rise times etc. However it also contains a circuit, typically a resistor divider and comparator to convert received LIN signals from the high to low voltage domains. The term high voltage transceiver will now be used to describe the LIN physical block in recognition that one such block can be used for all three interfaces. In addition the high voltage LIN physical block there is also a block referred to as a LIN digital block which is effectively a UART. Typically the LIN digital block converts serial data received from the LIN physical block into bytes of data which can be easily read by a uC or such and converts bytes of data from the uC into a serial format suitable for sending on the LIN physical. The LIN digital block would also take care of LIN sync, LIN break and the other protocol elements required by the LIN bus specification. e.g. CRC. The LIN digital block has a least one input RX and a least one output TX. The RX input normally comes from the LIN physical block and the TX output normally drives an input of the LIN physical block. The digital block will be different for the LIN, PWM and SENT interfaces. Hereafter only one block labeled “LIN DIGITAL” may be shown but the die may contain three or more user selectable digital blocks which can be selected to interface with the HV transceiver.
For example the primary circuit die could be an oil change indicator for monitoring pressure and temperature, while the secondary circuit die(s) monitor e.g., density, viscosity. The primary die interface method could be PWM, SENT or LIN. As secondary die(s) are added they generally need to communicate with the primary circuit die and/or external circuits. This requires a number of connections for power and communications. One way to maintain such communications between primary and secondary dies is with UARTs, but that requires a pair of UARTs for each secondary die. This provides for external communications to be received and processed by the primary circuit die then retransmitted to the designated secondary circuit die and that secondary circuit die must then process the communications and respond to the primary circuit die which then processes and transmits the communication to the external circuit originator. This requires many inter die connections between the primary and secondary dies. In addition the software on the primary die must be modified to control communications with the secondary dies. Software modification in itself either rules out or greatly exaggerates the use of a state machine as the control logic and requires something more flexible like a micro controller. The reduction in the number of interconnects is especially important if the two or more die are combined in one package and high temperatures or other aspects of a harsh environment disallow the use of a laminate or similar to facilitate multi layer interconnects. Alternatively, both the primary and each secondary circuit die could have high voltage processing circuits but that adds cost, complexity and requires more area.